Processes for producing acetic acid product having low butyl acetate content

ABSTRACT

A process for producing an acetic acid product having low butyl acetate content via a carbonylation reaction. The carbonylation reaction is carried out at a temperature from 100 to 300° C., a hydrogen partial pressure from 0.3 to 2 atm, and a metal catalyst concentration from 100 to 3000 wppm, based on the weight of the reaction medium. The butyl acetate concentration in the acetic acid product may be controlled by removing acetaldehyde from a stream derived from the reaction medium and/or by adjusting at least one of reaction temperature, hydrogen partial pressure, and metal catalyst concentration.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application claims priority from U.S. Provisional PatentApplication Ser. No. 62/079,991, entitled “Processes For Producing AnAcetic Acid Product Having Low Butyl Acetate Content”, filed Nov. 14,2014, the disclosure of which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to processes for producing an acetic acid product having lowbutyl acetate content.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one ofthe most useful commercially is the catalyzed carbonylation of methanolwith carbon monoxide as taught in U.S. Pat. No. 3,769,329, hereinincorporated by reference. The carbonylation catalyst contains a metalcatalyst, such as rhodium, which is either dissolved or otherwisedispersed in a liquid reaction medium or supported on an inert solid,along with a halogen-containing catalyst promoter as exemplified bymethyl iodide. The rhodium can be introduced into the reaction system inmany forms. Likewise, because the nature of the halide promoter is notgenerally critical, a large number of suitable promoters, most of whichare organic iodides, may be used. Most typically and usefully, thereaction is conducted by continuously bubbling carbon monoxide gasthrough a liquid reaction medium in which the catalyst is dissolved.

During the carbonylation of methanol, by-products are formed. Oneby-product is acetaldehyde. Reduction of acetaldehyde has been describedin the art. For example, U.S. Pat. No. 5,756,836 teaches a process forproducing a highly purified acetic acid characterized in that theprocess comprises the step of continuously reacting methanol and/or anaqueous solution of methyl acetate with carbon monoxide in a reactor. Atreatment is conducted to limit the concentration of unsaturatedcompounds in crude acetic acid obtained in the process to 5 wppm orlower, and the resultant crude acetic acid is ozonized. The '836 patentalso teaches a process for producing a highly purified acetic acid,characterized by the step of continuously reacting methanol and/or anaqueous solution of methyl acetate with carbon monoxide in a reactorwhile maintaining the concentration of acetaldehyde in a reaction fluidin the reactor at 1500 wppm or lower. The acetaldehyde concentration iscontrolled by conducting said reaction at a water content not greaterthan 10 wt. % and an acetaldehyde concentration of not greater than 1500wppm to produce a crude acetic acid product mixture; sending the crudeacetic acid product mixture to a distillation column to produce ahigh-boiling point fraction and a low-boiling point fraction; treatingthe low-boiling point fraction to reduce the content of acetaldehydetherein; and returning the treated low-boiling point fraction to thereaction system.

U.S. Pat. No. 5,625,095 also suggests that acetaldehyde concentrationshould be reduced. The '095 patent discloses a high purity acetic acidprepared by reacting methanol with carbon monoxide in the presence of arhodium catalyst, iodide salts, and methyl iodide, wherein anacetaldehyde concentration in the reaction liquid is maintained at 400wppm or lower. This may be attained by contacting the liquid containingcarbonyl impurities with water to separate and remove the carbonylimpurities. After that, the liquid can be returned to the reactor.

U.S. Pat. No. 6,573,403 teaches a process for producing acetic acidwhich comprises charging reactants methanol, dimethyl ether, methylacetate or any mixture thereof into a reactor containing: (1) a rhodiumcarbonylation catalyst, (2) an alkyl iodide or alkyl bromide, and (3) ahydrogenation catalyst, and contacting the reactants with carbonmonoxide and hydrogen to produce acetic acid. The '403 patent furtherteaches that the addition of ruthenium compounds to the carbonylationreaction solution conditions effectively reduces the formation ofundesired carbonyl impurities whilst increasing the formation ofethanol, ethyl acetate and ethyl iodide being precursors for theformation of valuable propanoic acid.

Additional methods for removing permanganate reducing compounds (PRC's),such as acetaldehyde, are disclosed in U.S. Pat. Nos. 7,855,306 and7,683,212. The '306 patent teaches a process for reducing and/orremoving permanganate reducing compounds or their precursors fromintermediate streams during the formation of acetic acid. In particular,a low boiling overhead vapor stream from a light ends column issubjected to a single distillation to obtain an overhead that issubjected to an extraction to selectively remove and/or reduce PRC'sfrom the process. The '212 patent teaches a method to produce aceticacid by continuously reacting methanol with carbon monoxide in thepresence of a rhodium catalyst, an iodide salt, methyl iodide, methylacetate, and water; and thereby producing acetic acid at a productionrate of 11 mol/L·hr or more while keeping the acetaldehyde content of areaction mixture to 500 wppm or less, in which the reaction is carriedout at a carbon monoxide partial pressure in a gaseous phase of areactor of 1.05 MPa or more and/or at a methyl acetate content of thereaction mixture of 2 percent by weight or more to thereby keep theproduction rate of acetaldehyde to 1/1500 or less that of acetic acid.The '212 patent teaches that this method can reduce production ofby-products without reducing the reaction rate of acetic acid even at alow water content and a low hydrogen partial pressure in a reactionsystem.

U.S. Pat. No. 6,303,813 discloses methanol carbonylation methods whichsubstantially reduce the production of carbonyl impurities, particularlyacetaldehyde, crotonaldehyde, and 2-ethyl crotonaldehyde, by maintaininga partial pressure of hydrogen between about 0.4 and 4 psia at reactiontotal pressure of from about 15 to about 40 atmospheres total reactionpressure.

Although the above-described publications focus on suppressing orremoving carbonyl impurities such as acetaldehyde and crotonaldehydefrom carbonylation reaction systems, little art exists concerning butylacetate, which can be formed from these impurities. The need thereforeexists for improved processes for producing a high purity acetic acidcomprising low amounts of butyl acetate.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a processfor producing an acetic acid product, comprising the steps ofcontinuously carbonylating at least one member selected from the groupconsisting of methanol, dimethyl ether, and methyl acetate with carbonmonoxide in a reactor in the presence of water, a metal catalyst, methyliodide and a halide salt to form a reaction medium, wherein thecarbonylating is carried out at a temperature from 150 to 250° C., ahydrogen partial pressure from 0.3 to 2 atm, and a metal catalystconcentration from 100 to 3000 wppm, based on the weight of the reactionmedium, removing acetaldehyde from a stream derived from the reactionmedium to form the acetic acid product, and maintaining a butyl acetateconcentration in the acetic acid product at 10 wppm or less. Thereaction medium may comprise less than 1500 wppm acetaldehyde. Thereaction medium may comprise from 0.1 to 3.5 wt. % water. The reactionmedium may comprise from 400 to 1500 wppm metal catalyst. In one aspect,the hydrogen partial pressure is from 0.3 to 1.5 atm. In another aspect,the hydrogen partial pressure is from 0.4 to 1.5 atm. The methanol maybe introduced into the reactor in a methanol source comprising from 1 to150 wppm ethanol. The acetic acid product may comprise less than 250wppm propionic acid. Removing acetaldehyde from a stream derived fromthe reaction medium may comprise: (a) separating at least a portion ofthe reaction medium to provide a vapor overhead stream comprising aceticacid and a less volatile catalyst phase; (b) distilling the vaporoverhead stream to yield a purified acetic acid product and a firstoverhead stream comprising methyl iodide, water, acetic acid, methylacetate, and acetaldehyde; (c) distilling at least a portion of thefirst overhead stream to form a second overhead stream and a liquidphase residuum, wherein the second overhead stream is enriched withacetaldehyde with respect to the at least a portion of the firstoverhead stream; and (d) extracting the second overhead stream withwater to obtain an aqueous acetaldehyde stream comprising acetaldehydeand a raffinate comprising methyl iodide. The methyl iodide from theraffinate may be returned, directly or indirectly, to the reactor. Theprocess may further comprise condensing and biphasically separating thefirst overhead stream to form a light liquid phase and a heavy liquidphase, wherein at least a portion of the light liquid phase is returnedto the reactor.

In a second embodiment, the present invention is directed to a processfor producing acetic acid, comprising: providing a reaction mediumcomprising acetic acid, methanol, methyl acetate, less than 4 wt. %water, a metal catalyst, methyl iodide and a halide organic salt;removing acetaldehyde from a stream derived from the reaction medium toform the acetic acid product, and maintaining a butyl acetateconcentration in the acetic acid product at 10 wppm or less. The butylacetate concentration may be maintained by removing acetaldehyde from astream derived from the reaction medium and by adjusting at least one ofreaction temperature, hydrogen partial pressure, and metal catalystconcentration in the reaction medium. In one aspect, the hydrogenpartial pressure is from 0.3 to 2 atm. In another aspect, the hydrogenpartial pressure is at least 0.4 atm. The removing acetaldehyde maycomprise: (a) separating at least a portion of the reaction medium toprovide a vapor overhead stream comprising acetic acid and a lessvolatile catalyst phase; (b) distilling the vapor overhead stream toyield a purified acetic acid product and a first overhead streamcomprising methyl iodide, water, acetic acid, methyl acetate, andacetaldehyde; (c) distilling at least a portion of the first overheadstream to form a second overhead stream and a liquid phase residuum,wherein the second overhead stream is enriched with acetaldehyde withrespect to the at least a portion of the first overhead stream; and (d)extracting the second overhead stream with water to obtain an aqueousacetaldehyde stream comprising acetaldehyde and a raffinate comprisingmethyl iodide. Methyl iodide from the raffinate may be returned,directly or indirectly, to the reactor. The process may further comprisecondensing and biphasically separating the first overhead stream to forma light liquid phase and a heavy liquid phase, wherein the at least aportion of the first overhead stream distilled in step (c) comprises thelight liquid phase. The process may further comprise condensing andbiphasically separating the first overhead stream to form a light liquidphase and a heavy liquid phase, wherein the at least a portion of thefirst overhead stream distilled in step (c) comprises the heavy liquidphase. The process acetic acid product may comprise less than 250 wppmpropionic acid.

In a third embodiment, the present invention is directed to a processfor producing an acetic acid product, comprising the steps of:continuously reacting methanol with carbon monoxide in the presence of arhodium catalyst, an iodide salt, and methyl iodide to form a reactionmedium, wherein the reaction is carried out at a temperature from 100 to300° C. and a hydrogen partial pressure from 0.3 to 2 atm, wherein thereaction medium comprises from 100 to 3000 wppm rhodium, separating theacetic acid product from the reaction medium; determining the butylacetate concentration in the acetic acid product; adjusting at least oneof the temperature, the hydrogen partial pressure and the rhodiumcatalyst concentration when the butyl acetate concentration in theacetic acid product is greater than 10 wppm; and removing acetaldehydefrom a stream derived from the reaction medium to maintain anacetaldehyde concentration of less than 1500 wppm in the reactionmedium.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures, wherein:

FIG. 1 shows a schematic of acetic acid reaction by-products andimpurities derived from acetaldehyde;

FIG. 2 shows a schematic of an acetic acid production process inaccordance with the present invention;

FIG. 3 shows a time series plot of butyl acetate in a final acetic acidproduct in accordance with the present invention;

FIG. 4 shows a another time series plot of butyl acetate in a finalacetic acid product in accordance with the present invention; and

FIG. 5 shows yet another time series plot of butyl acetate in a finalacetic acid product in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION I. Introduction

The present invention relates to processes for producing acetic acid,comprising carbonylating at least one member selected from the groupconsisting of methanol, dimethyl ether and methyl acetate in thepresence of a water, a metal catalyst, methyl iodide and a halide saltto form a reaction medium. The carbonylating is carried out at atemperature from 150 to 250° C., a hydrogen partial pressure from 0.3 to2 atm, and a metal catalyst concentration from 100 to 3000 wppm based onthe weight of the reaction medium, while maintaining a butyl acetateconcentration in the high purity acetic at 10 wppm or less. Due to thedifficulty in removing butyl acetate from acetic acid in a carbonylationpurification process, butyl acetate concentration in the acetic acidproduct is not easily controlled. Butyl acetate is not a PRC, but it isan impurity that can lead to degradation of acetic acid product qualityin concentrations above 10 wppm. The butyl acetate concentration in thefinal high purity acetic acid product is at least partially maintainedby removing acetaldehyde from a stream derived from the reaction medium.The butyl acetate concentration in the final high purity acetic acidproduct may also be maintained by adjusting at least one of the reactiontemperature, the hydrogen partial pressure, the metal catalystconcentration, and the water concentration. It has been surprisingly andunexpectedly discovered that by removing acetaldehyde from a streamderived from the reaction medium and by controlling at least one ofreaction temperature, hydrogen partial pressure, and metal catalystconcentration in the reaction medium, the amount of butyl acetate in thefinal acetic acid product can be advantageously maintained at 10 wppm orless. As a consequence of these controls, the amount of propionic acidin the final acetic acid product can be maintained at 250 wppm or less.

In addition to the carbonylation reaction, several side reactions occurin the reaction medium. Without being bound by theory, FIG. 1 showsvarious by-products and impurities that may be formed in a carbonylationprocess by hydrogenation and aldol condensation reactions. Whenacetaldehyde is present in the reaction medium, the acetaldehyde isconverted to crotonaldehyde via an aldol condensation reaction.Crotonaldehyde may then be hydrogenated to butyl aldehyde, which maythen be hydrogenated to butanol. Finally, butanol may react with aceticacid to form butyl acetate. In addition to butyl aldehyde beinghydrogenated to butanol, butyl aldehyde may react with acetaldehyde toform additional impurities, as shown in FIG. 1. As shown in FIG. 1,acetaldehyde concentration is only one factor affecting butyl acetateconcentration in the final acetic acid product. Thus, acetaldehydeconcentration in the reaction medium is not an accurate predictor, byitself, of final butyl acetate content and other parameters may becontrolled to achieve the desired butyl acetate content in the finalacetic acid product.

II. Carbonylation Reaction Step

Exemplary reaction and acetic acid recovery system 100 is shown in FIG.2. As shown, methanol-containing feed stream 101 and carbonmonoxide-containing feed stream 102 are directed to liquid phasecarbonylation reactor 105, in which the carbonylation reaction occurs toform acetic acid.

Methanol-containing feed stream 101 may comprise at least one memberselected from the group consisting of methanol, dimethyl ether, andmethyl acetate. Methanol-containing feed stream 101 may be derived inpart from a fresh feed or may be recycled from the system. At least someof the methanol and/or reactive derivative thereof will be converted to,and hence be present as, methyl acetate in the liquid medium byesterification reaction with acetic acid.

Typical reaction temperatures for carbonylation will be from 150 to 250°C., with the temperature range of 180 to 225° C. being a preferredrange. The carbon monoxide partial pressure in the reactor can varywidely but is typically from 2 to 30 atm, e.g., from 3 to 10 atm. Thehydrogen partial pressure in the reactor is typically from 0.3 to 2 atm,e.g., from 0.3 to 1.5 atm, or from 0.4 to 1.5 atm. Because of thepartial pressure of by-products and the vapor pressure of the containedliquids, the total reactor pressure will range from 15 to 40 atm. Theproduction rate of acetic acid may be from 5 to 50 mol/L·h, e.g., from10 to 40 mol/L·h, and preferably 15 to 35 mol/L·h.

Carbonylation reactor 105 is preferably either a mechanically stirredvessel, a vessel with educated or pump-around mixing, or bubble-columntype vessel, with or without an agitator, within which the reactingliquid or slurry contents are maintained, preferably automatically, apredetermined level, which preferably remains substantially constantduring normal operation. Into carbonylation reactor 105, fresh methanol,carbon monoxide, and sufficient water are continuously introduced asneeded to maintain suitable concentrations in the reaction medium.

The metal catalyst may comprise a Group VIII metal. Suitable Group VIIIcatalysts include rhodium and/or iridium catalysts. When a rhodiumcatalyst is used, the rhodium catalyst may be added in any suitable formsuch that rhodium is in the catalyst solution as an equilibrium mixtureincluding [Rh(CO)₂I₂]-anion, as is well known in the art. Iodide saltsoptionally maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal, quaternary ammonium, phosphonium salt ormixtures thereof. In certain embodiments, the catalyst co-promoter islithium iodide, lithium acetate, or mixtures thereof. The saltco-promoter may be added as a non-iodide salt that will generate aniodide salt. The iodide catalyst stabilizer may be introduced directlyinto the reaction system. Alternatively, the iodide salt may begenerated in-situ since under the operating conditions of the reactionsystem, a wide range of non-iodide salt precursors will react withmethyl iodide or hydroiodic acid in the reaction medium to generate thecorresponding co-promoter iodide salt stabilizer. For additional detailregarding rhodium catalysis and iodide salt generation, see U.S. Pat.Nos. 5,001,259; 5,026,908; 5,144,068 and 7,005,541, which areincorporated herein by reference in their entirety. The carbonylation ofmethanol utilizing iridium catalyst is well known and is generallydescribed in U.S. Pat. Nos. 5,942,460, 5,932,764, 5,883,295, 5,877,348,5,877,347 and 5,696,284, which are incorporated herein by reference intheir entirety.

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter will include methyl halide, and more preferably methyliodide.

The components of the reaction medium are maintained within definedlimits to ensure sufficient production of acetic acid. The reactionmedium contains a concentration of the metal catalyst, e.g. rhodiumcatalyst, in an amount from 100 to 3000 wppm, e.g., from 400 to 2000wppm, or from 400 to 1500 wppm as rhodium. The concentration of water inthe reaction medium is maintained to be less than 14 wt. %, e.g., from0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. % or from 0.25 wt. % to5 wt. %. Preferably, the reaction is conducted under low waterconditions and the reaction medium contains less than 4 wt. % water,e.g., less than 3.5 wt. %, less than 3 wt. %, or less than 2 wt. %. Interms of ranges, the reaction medium contains 0.1 to 3.5 wt. % water,e.g, from 0.1 to 3 wt. % or from 0.5 to 2.8 wt. %. The concentration ofmethyl iodide in the reaction medium is maintained to be from 1 to 25wt. %, e.g., from 5 to 20 wt. %, from 4 to 13.9 wt. %. The concentrationof iodide salt, e.g., lithium iodide, in the reaction medium ismaintained to be from 1 to 25 wt. %, e.g., from 2 to 20 wt. %, from 3 to20 wt. %. The concentration of methyl acetate in the reaction medium ismaintained to be from 0.5 to 30 wt. %, e.g., from 0.3 to 20 wt. %, from0.6 to 4.1 wt. %. The following amounts are based on the total weight ofthe reaction medium. The ranges disclosed in this application includethe endpoints, subranges and individual values.

The concentration of acetic acid in the reaction medium is generallymore than 30 wt. %, e.g. more than 40 wt. % or more than 50 wt. %.

In some embodiments, the desired reaction rates are obtained even at lowwater concentrations by maintaining in the reaction medium an ester ofthe desired carboxylic acid and an alcohol, desirably the alcohol usedin the carbonylation, and an additional iodide ion that is over andabove the iodide ion that is present as hydrogen iodide. A desired esteris methyl acetate. The additional iodide ion is desirably an iodidesalt, with lithium iodide (LiI) being preferred. It has been found, asdescribed in U.S. Pat. No. 5,001,259, that under low waterconcentrations, methyl acetate and lithium iodide act as rate promotersonly when relatively high concentrations of each of these components arepresent and that the promotion is higher when both of these componentsare present simultaneously.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing therhodium catalyst, methyl iodide (MeI) promoter, methyl acetate (MeAc),and additional soluble iodide salt, at conditions of temperature andpressure suitable to form the carbonylation product. It will begenerally recognized that it is the concentration of iodide ion in thecatalyst system that is important and not the cation associated with theiodide, and that at a given molar concentration of iodide the nature ofthe cation is not as significant as the effect of the iodideconcentration. Any metal iodide salt, or any iodide salt of any organiccation, or other cations such as those based on amine or phosphinecompounds (optionally quaternary cations), can be maintained in thereaction medium provided that the salt is sufficiently soluble in thereaction medium to provide the desired level of the iodide. When theiodide is a metal salt, preferably it is an iodide salt of a member ofthe group consisting of the metals of Group IA and Group IIA of theperiodic table as set forth in the “Handbook of Chemistry and Physics”published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). Inparticular, alkali metal iodides are useful, with lithium iodide beingparticularly suitable. In the low water carbonylation process, theadditional iodide ion over and above the iodide ion present as hydrogeniodide is generally present in the catalyst solution in amounts suchthat the total iodide ion concentration is from 1 to 25 wt. % and themethyl acetate is generally present in amounts from 0.5 to 30 wt. %, andthe methyl iodide is generally present in amounts from 1 to 25 wt. %.The rhodium catalyst is generally present in amounts from 200 to 3000wppm.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably below the agitator,which may be used to stir the contents. The gaseous feed preferably isthoroughly dispersed through the reacting liquid by this stirring means.Gaseous purge stream 106 desirably is vented from the reactor 105 toprevent buildup of gaseous by-products and to maintain a set carbonmonoxide partial pressure at a given total reactor pressure. Thetemperature of the reactor may be controlled and the carbon monoxidefeed is introduced at a rate sufficient to maintain the desired totalreactor pressure. Stream 105 comprising the liquid reaction medium exitsreactor 105.

The acetic acid production system preferably includes separation system108 employed to recover the acetic acid and recycle metal catalyst,methyl iodide, methyl acetate, and other system components within theprocess. One or more of the recycle streams may be combined prior tobeing introduced into the reactor. The separation system also preferablycontrols water and acetic acid content in the carbonylation reactor, aswell as throughout the system, and facilitates permanganate reducingcompound (“PRC”) removal. PRC's may include acetaldehyde, acetone,methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, 2-ethyl butyraldehyde, and the aldol condensationproducts thereof.

The reaction medium is drawn off from the carbonylation reactor 105 at arate sufficient to maintain a constant level therein and is provided toflasher 110 via stream 113. The flash separation may be carried out at atemperature from 80° C. to 200° C., under an absolute pressure from 1 to10 atm. In flasher 110, the reaction medium is separated in a flashseparation step to obtain a vapor product stream 112 comprising aceticacid and liquid recycle 111 comprising a catalyst-containing solution.

In addition to acetic acid, vapor product stream 112 also comprisesmethyl iodide, methyl acetate, water, PRC's. Dissolved gases exitingreactor 105 and entering flasher 110 comprise a portion of the carbonmonoxide and may also contain gaseous by-products such as methane,hydrogen, and carbon dioxide. Such dissolved gases exit flasher 110 aspart of the vapor product stream 112. In one embodiment, carbon monoxidein gaseous purge stream 106 is fed to the base of flasher 110 to enhancerhodium stability. The catalyst-containing solution in liquid recycle111 may be predominantly acetic acid and also contain the rhodium andthe iodide salt along with lesser quantities of methyl acetate, methyliodide, and water. The catalyst-containing solution in liquid recycle111 is recycled to the reactor, as discussed above.

III. Butyl Acetate Concentration in the Final Acetic Acid Product

As described herein, a acetic acid product, preferably a high purityacetic acid product, is formed by the processes of the presentinvention, preferably having a butyl acetate concentration of 10 wppm orless, e.g., 9 wppm or less, 8 wppm or less, 6 wppm or less, 2 wppm orless or substantially free of butyl acetate, e.g., non-detectable. Interms of ranges, the acetic acid product may have a butyl acetatecontent from 0 to 10 wppm, e.g., from 0.1 to 9 wppm, from 0.2 to 8 wppm,from 0.3 to 6 wppm, or from 0.5 to 2 wppm. The high purity acetic acidproduct also preferably has a propionic acid concentration of less than250 wppm propionic acid, e.g., less than 225 wppm or less than 200 wppm.One variable that may be adjusted to control the butyl acetateconcentration of the high purity acetic acid product is the acetaldehydeconcentration in the reaction medium. As disclosed herein, butyl acetateis a by-product formed from acetic acid and butanol, which is derivedultimately from acetaldehyde as shown in FIG. 1. Thus, as acetaldehydeconcentration is decreased, butyl acetate concentration is generallydecreased. Acetaldehyde removal systems are described further herein,including in FIG. 2. Preferably, the acetaldehyde concentration in thereaction medium is maintained at less than 1500 wppm acetaldehyde, e.g.,less than 900 wppm, less than 500 wppm, or less than 400 wppm.

The acetaldehyde concentration in the reaction medium may be controlledby removing acetaldehyde from a stream derived from the reaction medium.This includes streams that are intended to be recycled to the reactorand that are derived from the vapor overhead stream, but excludes theacetic acid product stream.

In addition to removing acetaldehyde from a stream derived from thereaction medium, it has now been discovered that butyl acetateconcentration in the final acetic acid product may be controlled byadjusting at least one of reaction temperature, hydrogen partialpressure, and metal catalyst concentration in the reaction medium. Thereaction temperature may be adjusted within the range of 150 to 250° C.,e.g., within 180 to 225° C. Hydrogen partial pressure may be adjustedwithin the range of 0.3 to 2 atm, e.g., from 0.3 to 1.5 atm, from 0.4 to1.5 atm, or from 0.3 to 1 atm. In some aspects, the hydrogen partialpressure is at least 0.3 atm, e.g., at least 0.35 atm, at least 0.4 atmor at least 0.5 atm. It is understood that 1 atm is equivalent toapproximately 101.33 kPa and 14.70 psi. As hydrogen partial pressure isincreased, the water-gas shift reaction of carbon monoxide and water tocarbon dioxide and hydrogen is affected since the carbon dioxidecomponent is decreased. Increasing the hydrogen partial pressure alsoallows for the reduction in temperature, leading to reduced operatingcosts. Finally, increasing the hydrogen partial pressure improves metalcatalyst activity, e.g., rhodium activity, by shifting the reactionequilibria to more rhodium in the active form. However, as hydrogenpartial pressure is increased, impurity production is also increased.Thus, hydrogen partial pressure and reaction temperature are balanced toachieve satisfactory yields, costs, and impurity concentrations. Thehydrogen partial pressure may be adjusted by modifying the amount ofhydrogen in the carbon monoxide source or by increasing reactor ventflows.

Based on the reaction mechanisms disclosed in FIG. 1, the concentrationof propionic acid may be affected by the concentration butyl acetate inthe acetic acid product. The concentration of propionic acid may beaffected by other variables, including the concentration ofacetaldehyde, ethanol content in the methanol source, hydrogen partialpressure, hydrogen content in the carbon monoxide source and reactionpressure. Ethanol may be present as an impurity in the methanol source,which may comprise from 1 to 150 wppm ethanol, e.g., from 1 to 100 wppm,from 1 to 50 wppm or from 1 to 25 wppm. The ethanol concentration in themethanol source may vary. Optionally, the methanol source is purified toincrease methanol content and reduce ethanol content prior to feeding tothe carbonylation reactor. Therefore, the ethanol concentration in themethanol source may be less than 1 wppm, e.g., free of ethanol.

IV. Recovery of Acetic Acid

The distillation and recovery of acetic acid is not particularly limitedfor the purposes of the present invention. In contrast to previousmethods that recover acetic acid from the vapor product stream, thepresent invention recovers acetic acid from both the vapor productstream and a liquid stream condensed from the vapor product stream thatis enriched in acetic acid.

As shown in FIG. 2, vapor product stream 112 is directed to a firstcolumn 120, also referred to as a light ends column. Distillation yieldsa low-boiling overhead vapor stream 122, a purified acetic acid productthat preferably is removed via a side stream 124, and a high boilingresidue stream 121. In one embodiment, low-boiling overhead vapor stream122 comprises from 40 to 80 wt. % water, methyl acetate, methyl iodide,and carbonyl impurities including acetaldehyde. Side stream 124 maycomprise from 85 to 98 wt. % acetic acid, from 1 to 5 wt. % water, from0.1 to 5 wt. % methyl iodide, and from 0.1 to 5 wt. % methyl acetate.Acetic acid removed via side stream 124 preferably is subjected tofurther purification, such as in a second column 125, also referred toas a drying column, and separates side stream 124 into overhead stream126 comprised primarily of water and bottoms stream 127 comprisedprimarily of acetic acid, e.g., the acetic acid product. Propionic acidin column 125 is concentrated with the acetic acid product in an amountof less than 250 wppm and is not removed from the acetic acid product.In some embodiments, the acetic acid product may be taken as a sidestream (not shown) from column 125. Advantageously, this avoids the needfor an additional separation step for removing propionic acid fromacetic acid. For example, no heavy ends removal is needed.

Overhead stream 126 may comprise 50 to 75 wt. % water. Methyl acetateand methyl iodide are also removed from the side stream and concentratedin the overhead stream. Drying column bottoms stream 127 preferablycomprises acetic acid. In preferred embodiments, drying column bottomsstream 127 comprises acetic acid in an amount greater than 90 wt. %,e.g., greater than 95 wt. % or greater than 98 wt. % and comprises lessthan 250 wppm propionic acid. Drying column bottoms stream 127 may befurther processed, e.g. by passing through an ion exchange resin, priorto being stored or transported for commercial use.

Low-boiling overhead vapor stream 122 separated from first column 120contains a reaction component, such as methyl iodide, methyl acetate,and water, and it is preferable to retain these reaction componentswithin the process. Low-boiling overhead vapor stream 122 is condensedin a heat exchanger into stream 133. At least a portion of stream 133may be directed to a PRC's removal unit 131, discussed herein.Optionally, a portion of stream 133 is recycled to reactor 105 and/orrefluxed first column 120. Similarly, overhead stream 126 from secondcolumn 125 contains a reaction component, such as methyl iodide, methylacetate, and water, and it is preferable to retain these reactioncomponents within the process. Overhead stream 126 is condensed in aheat exchanger into stream 136, which is recycled to reactor 105 and/orrefluxed second column 125. An offgas component may be vented via line135 from condensed low-boiling overhead vapor stream 126. Similar to thecondensed low-boiling overhead vapor stream in stream 133, condensedoverhead stream in stream 136 may also be separated into an aqueousphase and an organic phase, and these phases may be recycled or refluxedas needed to maintain the concentrations in the reaction medium.

To recover residue liquids from the vent stream, in particular lines106, 132, 135, and 122, these lines may be fed to a scrubber thatoperates with cooled methanol and/or acetic acid to remove methylacetate and methyl iodide. A suitable scrubber is described in U.S. Pat.No. 8,318,977, which is incorporated herein by reference in itsentirety.

The distillation columns of the present invention may be conventionaldistillation column, e.g., a plate column, a packed column, and others.The material of the distillation column is not limited and may include aglass, a metal, a ceramic, or other suitable material can be used. For aplate column, the theoretical number of plates may depend on thecomponent to be separated, and may include up to 50 plates, e.g., from 5to 50, or from 7 to 35.

V. PRC Removal System (PRS)

The PRS may contain a single extraction step or may include multipleextraction stages, as described for example in U.S. Pat. No. 7,223,886and optionally including multistage countercurrent extraction. Accordingto various embodiments, one or more streams derived from either or both(i) the PRS distillation column and/or (ii) the PRS extraction stage,for example, may be returned to the system, e.g., either or both (i) thelight ends removal column and/or (ii) the drying column of theseparation system for the acetic acid production system. For example, afirst portion, e.g., an aliquot portion, of a bottoms stream from a PRScolumn may be directed to light ends column 120 for further processing,or a second portion, e.g., an aliquot portion, of a bottoms stream froma PRS column may be directed to drying column 125, preferably the upperportion of drying column 125, for further processing. As anotherexample, a raffinate from a PRS extraction unit, notably containingmethyl iodide, may be returned to the system, e.g., light ends column ordrying column or the raffinate may be added directly to decanter 140and/or may be returned to reactor 105.

For purposes of the present specification and claims, the overheadstreams and overhead decanters of the light ends removal column and thedrying column are considered to be part of the light ends removal columnand of the drying column.

As indicated above, either phase of the low-boiling overhead vaporstream 133 may be subsequently processed to remove PRC's. Thus, eitherphase of stream 133 may be referred to as a stream derived from thereaction medium.

For purposes of the present specification, it should be understood thatthe term “aliquot portion” refers to both: (i) a portion of a parentstream that has the same composition as the parent stream from which itis derived, and (ii) a stream comprising a portion of a parent streamthat has the same composition as the parent stream from which it isderived and one or more additional streams that have been combinedtherewith. Thus, directing a return stream comprising an aliquot portionof a PRS distillation bottoms stream to the light ends columnencompasses the direct transfer of a portion of the PRS distillationbottoms stream to the light ends column as well as the transfer of aderivative stream comprising (i) a portion of the PRS distillationbottoms stream and (ii) one or more additional streams that are combinedtherewith prior to introduction into the light ends column. An “aliquotportion” would not include, for example, streams formed in adistillation step or a phase separation step, which would not becompositionally the same as the parent stream from which they arederived nor derived from such a stream.

One of ordinary skill in the art having the benefit of this disclosurecan design and operate a PRS distillation column to achieve the desiredresults. Accordingly, the practice of this process is not necessarilylimited to specific characteristic of a particular distillation columnor the operation characteristics thereof, such as the total number ofstages, the feed point, reflux ratio, feed temperature, refluxtemperature, column temperature profile, and the like.

In some cases, it may be advantageous to remove PRC's, primarilyaldehydes such as acetaldehyde, from a low-boiling overhead vapor streamof a light ends distillation column, more preferably from the condensedlight phase of a low-boiling overhead vapor stream 133 from light endsdistillation column 120.

One or more of the streams from PRS 131 may be returned to the system,e.g., recycled, either directly or indirectly. The PRS preferablyincludes at least one distillation column and at least one extractioncolumn to reduce and/or remove PRCs. US Patent Publication No.2011/0288333, which is hereby incorporated by reference, describesvarious PRS embodiments that may be employed with the present process.

In one embodiment, as shown in FIG. 2, PRS 131 comprises a column 145,an accumulator 150, and an extractor 155. At least a portion oflow-boiling overhead vapor stream 133 is directed to decanter 140 toform a heavy phase stream 141, and a light phase 142. Optionally, aportion of stream 142 is returned to column 120 via stream 142′.Additionally, a portion of heavy phase 141 may be returned to reactor105. Optionally, a slip stream (not shown), e.g., from 5 to 40 vol. % orfrom 5 to 20 vol. % of heavy phase 141 is directed to PRS 131. An offgascomponent may be vented via line 132 from decanter 140.

At least a portion of light phase 142 is directed to column 145 to forma vapor overhead stream 146 and a bottom process stream 147 comprisingwater, methyl acetate, methanol, and mixtures thereof. Vapor overheadstream 146 is passed through a condenser and collected in accumulator150. A portion of the condensed vapor overhead stream may be returned tocolumn 145 via line 151. Another portion of the condensed vapor overheadstream is directed to extractor 155 via line 152 to form waste stream156 comprising at least one PRC, e.g., acetaldehyde, and process stream157 comprising methyl iodide. An aqueous stream may be provided toextractor 155 via line 158 at a location to obtain a countercurrentflow.

Dimethyl ether (“DME”) may be present in the PRS in amount sufficient toreduce the solubility of methyl iodide in the aqueous extracted phase.Reducing the amount of methyl iodide in the aqueous extracted phasereduces losses of methyl iodide into waste stream 156. In someembodiments, this may allow multiple extractions as described in U.S.Pat. Nos. 7,223,886, and 8,076,507, the entireties of which are hereinincorporated by reference. The amount of DME may vary depending on themethyl iodide concentrations, and in some embodiments, the amount of DMEmay range from 3 to 9 wt. %, e.g. from 4 to 8 wt. %. DME may be presentin the PRS, formed in the PRS by adding water to the PRS (generally byadding water to column 145) or added to the PRS (generally by adding DMEupstream of extractor 155).

VI. Guard bed

Carboxylic acid streams, e.g., acetic acid streams, that arecontaminated with a halides and/or corrosion metals may be contactedwith an ion exchange resin composition under a wide range of operatingconditions. Preferably, the ion exchange resin composition is providedin a guard bed. The use of guard beds to purify contaminated carboxylicacid streams is well documented in the art, for example, U.S. Pat. Nos.4,615,806; 5,653,853; 5,731,252; and 6,225,498, which are herebyincorporated by reference in their entireties. Generally, a contaminatedliquid carboxylic acid stream is contacted with the inventive ionexchange resin composition, which is preferably disposed in the guardbed. The halide contaminants, e.g., iodide contaminants, react with themetal to form metal iodides. In some embodiments, hydrocarbon moieties,e.g., methyl groups, that may be associated with the iodide may esterifythe carboxylic acid. For example, in the case of acetic acidcontaminated with methyl iodide, methyl acetate would be produced as abyproduct of the iodide removal. The formation of this esterificationproduct typically does not have a deleterious effect on the treatedcarboxylic acid stream.

The pressure during the contacting step is limited only by the physicalstrength of the resin. In one embodiment, the contacting is conducted atpressures ranging from 0.1 MPa to 1 MPa, e.g., from 0.1 MPa to 0.8 MPaor from 0.1 MPa to 0.5 MPa. For convenience, however, both pressure andtemperature preferably may be established so that the contaminatedcarboxylic acid stream is processed as a liquid. Thus, for example, whenoperating at atmospheric pressure, which is generally preferred based oneconomic considerations, the temperature may range from 17° C. (thefreezing point of acetic acid) and 118° C. (the boiling point of aceticacid). It is within the purview of those skilled in the art to determineanalogous ranges for product streams comprising other carboxylic acidcompounds. The temperature of the contacting step preferably is keptrelatively low to minimize resin degradation. In one embodiment, thecontacting is conducted at a temperature ranging from 25° C. to 120° C.,e.g., from 25° C. to 100° C. or from 50° C. to 100° C. Some cationicmacroreticular resins typically begin degrading (via the mechanism ofacid-catalyzed aromatic desulfonation) at temperatures of 150° C.Carboxylic acids having up to 5 carbon atoms, e.g., up to 3 carbonatoms, remain liquid at these temperatures. Thus, the temperature duringthe contacting should be maintained below the degradation temperature ofthe resin utilized. In some embodiments, the operating temperature iskept below temperature limit of the resin, consistent with liquid phaseoperation and the desired kinetics for halide removal.

The configuration of the guard bed within an acetic acid purificationtrain may vary widely. For example, the guard bed may be configuredafter a drying column. Additionally or alternatively, the guard be maybe configured after a heavy ends removal column or finishing column.Preferably the guard bed is configured in a position wherein thetemperature acetic acid product stream is low, e.g., less than 120° C.or less than 100° C. Aside from the advantages discussed above, lowertemperature operation provides for less corrosion as compared to highertemperature operation. Lower temperature operation provides for lessformation of corrosion metal contaminants, which, as discussed above,may decrease overall resin life. Also, because lower operatingtemperatures result in less corrosion, vessels advantageously need notbe made from expensive corrosion-resistant metals, and lower grademetals, e.g., standard stainless steel, may be used.

In one embodiment, the flow rate through the guard bed ranges from 0.1bed volumes per hour (“BV/hr”) to 50 BV/hr, e.g., 1 BV/hr to 20 BV/hr orfrom 6 BV/hr to 10 BV/hr. A bed volume of organic medium is a volume ofthe medium equal to the volume occupied by the resin bed. A flow rate of1 BV/hr means that a quantity of organic liquid equal to the volumeoccupied by the resin bed passes through the resin bed in a one hourtime period.

To avoid exhausting the resin with a purified acetic acid product thatis high in total iodide concentration, in one embodiment the purifiedacetic acid product in bottoms stream 127 is contacted with a guard bedwhen total iodide concentration of the purified acetic acid product isless than 1 wppm. Total iodide concentration includes iodide from bothorganic, C₁ to C₁₄ alkyl iodides, and inorganic sources, such ashydrogen iodide. A purified acetic acid composition is obtained as aresult of the guard bed treatment. The purified acetic acid composition,in one embodiment, comprises less than 100 wppb, iodides, e.g., lessthan 90 wppb, less than 50 wppb, or less than 25 wppb. In oneembodiment, the purified acetic acid composition comprises less than 100wppb corrosion metals, e.g., less than 750 wppb, less than 500 wppb, orless than 250 wppb. In terms of ranges, the purified acetic acidcomposition may comprise from 0 to 100 wppb iodides, e.g., from 1 to 50wppb; and/or from 0 to 1000 wppb corrosion metals, e.g., from 1 to 500wppb. In other embodiments, the guard bed removes at least 25 wt % ofthe iodides from the crude acetic acid product, e.g., at least 50 wt %or at least 75 wt %. In one embodiment, the guard bed removes at least25 wt % of the corrosion metals from the crude acetic acid product,e.g., at least 50 wt % or at least 75 wt %.

VII. Examples

The present invention will be better understood in view of the followingnon-limiting examples.

Example 1

A final acetic acid product was prepared as follows. The reaction mediumcontained 10.8 wt. % methyl iodide, 2.5 wt. % methyl acetate, 4.1 wt. %water, 657 wppm rhodium and 8.4 wt. % lithium iodide. The reactiontemperature was 191.8° C. The hydrogen partial pressure was 0.46 atm.The reaction medium was flashed and then separated in a light endscolumn, PRS, and drying column as described herein. The concentration ofbutyl acetate in the final acetic acid product was measured over time.The results are shown in FIG. 3. Butyl acetate concentration wasconsistently below 8 wppm in the final acetic acid product.

Example 2

A final acetic acid product was prepared as in Example 1, except thatthe hydrogen partial pressure was 0.30 atm. The concentration of butylacetate in the final acetic acid product was measured over time. Theresults are shown in FIG. 4. Butyl acetate concentration wasconsistently below 6 wppm in the final acetic acid product.

Example 3

A final acetic acid product was prepared as in Example 1, except thatthe hydrogen partial pressure was 0.61 atm. The concentration of butylacetate in the final acetic acid product was measured over time. Theresults are shown in FIG. 5. Butyl acetate concentration wasconsistently below 2 wppm in the final acetic acid product.

Example 4

A final acetic acid product was prepared as in Example 1, except thatthe reaction medium contained approximately 7 wt. % methyl iodide,approximately 2 wt. % methyl acetate, approximately 2 wt. % water,approximately 1500 wppm rhodium and approximately 9 wt. % to 10 wt. %lithium iodide. The reaction temperature was approximately 200° C. Thehydrogen partial pressure was 0.44 atm. Butyl acetate concentration wasconsistently below 10 wppm in the final acetic acid product.

Comparative Example A

As shown in Example 3 of U.S. Pat. No. 6,303,813, a final acetic acidproduct was prepared as follows. The reaction medium contained 10.8 wt.% methyl iodide, 2.5 wt. % methyl acetate, 4.1 wt. % water, 657 wppmrhodium and 8.4 wt. % lithium iodide. The reaction temperature was191.8° C. The hydrogen partial pressure was 0.46 atm. The reactionmedium was flashed and then separated in a light ends column, and dryingcolumn as described herein. No PRS was used. The concentration of butylacetate in the final acetic acid product 13 wppm.

Comparative Example B

As shown in Example 4 of U.S. Pat. No. 6,303,813, a final acetic acidproduct was prepared as follows. The reaction medium contained 10.8 wt.% methyl iodide, 2.5 wt. % methyl acetate, 4.1 wt. % water, 657 wppmrhodium and 8.4 wt. % lithium iodide. The reaction temperature was191.8° C. The hydrogen partial pressure was 0.30 atm. The reactionmedium was flashed and then separated in a light ends column, and dryingcolumn as described herein. No PRS was used. The concentration of butylacetate in the final acetic acid product 16 wppm.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for producing an acetic acid product comprisingthe steps of continuously carbonylating at least one member selectedfrom the group consisting of methanol, dimethyl ether, and methylacetate with carbon monoxide in a reactor in the presence of water, arhodium catalyst, methyl iodide and a halide salt to form a reactionmedium, wherein the carbonylating is carried out at a temperature from150 to 250° C., a hydrogen partial pressure from 0.3 to 2 atm, and arhodium catalyst concentration from 100 to 3000 wppm, based on theweight of the reaction medium, removing acetaldehyde from a streamderived from the reaction medium to form the acetic acid product, andmaintaining a butyl acetate concentration in the acetic acid product at10 wppm or less.
 2. The process of claim 1, wherein the reaction mediumcomprises less than 1500 wppm acetaldehyde.
 3. The process of claim 1,wherein water is present from 0.1 to 3.5 wt. %.
 4. The process of claim1, wherein the reaction medium comprises from 400 to 1500 wppm metalcatalyst.
 5. The process of claim 1, wherein the hydrogen partialpressure is from 0.3 to 1.5 atm.
 6. The process of claim 1, wherein thehydrogen partial pressure is from 0.4 to 1.5 atm.
 7. The process ofclaim 1, wherein the methanol is introduced into the reactor in amethanol source comprising from 1 to 150 wppm ethanol.
 8. The process ofclaim 1, wherein the acetic acid product comprises less than 250 wppmpropionic acid.
 9. The process of claim 1, wherein removing acetaldehydefrom a stream derived from the reaction medium comprises: (a) separatingat least a portion of the reaction medium to provide a vapor overheadstream comprising acetic acid and a less volatile catalyst phase; (b)distilling the vapor overhead stream to yield a purified acetic acidproduct and a first overhead stream comprising methyl iodide, water,acetic acid, methyl acetate, and acetaldehyde; (c) distilling at least aportion of the first overhead stream to form a second overhead streamand a liquid phase residuum, wherein the second overhead stream isenriched with acetaldehyde with respect to the at least a portion of thefirst overhead stream; and (d) extracting the second overhead streamwith water to obtain an aqueous acetaldehyde stream comprisingacetaldehyde and a raffinate comprising methyl iodide.
 10. The processof claim 9, further comprising condensing and biphasically separatingthe first overhead stream to form a light liquid phase and a heavyliquid phase, wherein the at least a portion of the first overheadstream distilled in step (c) comprises the light liquid phase.
 11. Theprocess of claim 9, further comprising condensing and biphasicallyseparating the first overhead stream to form a light liquid phase and aheavy liquid phase, wherein the at least a portion of the first overheadstream distilled in step (c) comprises the heavy liquid phase.
 12. Aprocess for producing acetic acid, comprising: providing a reactionmedium comprising acetic acid, methanol, methyl acetate, water, arhodium catalyst, methyl iodide and a halide organic salt; removingacetaldehyde from a stream derived from the reaction medium to form theacetic acid product; and maintaining a butyl acetate concentration inthe acetic acid product at 10 wppm or less; wherein the water is presentin the reaction medium at less than 4 wt. % and the reaction is carriedout at a hydrogen partial pressure from 0.3 to 2 atm.
 13. The process ofclaim 12, wherein the butyl acetate concentration is maintained byremoving acetaldehyde from a stream derived from the reaction medium andby adjusting at least one of reaction temperature, hydrogen partialpressure, and metal catalyst concentration in the reaction medium. 14.The process of claim 13, wherein the hydrogen partial pressure is atleast 0.4 atm.
 15. The process of claim 13, wherein the removingacetaldehyde comprises: (a) separating at least a portion of thereaction medium to provide a vapor overhead stream comprising aceticacid and a less volatile catalyst phase; (b) distilling the vaporoverhead stream to yield a purified acetic acid product and a firstoverhead stream comprising methyl iodide, water, acetic acid, methylacetate, and acetaldehyde; (c) distilling at least a portion of thefirst overhead stream to form a second overhead stream and a liquidphase residuum, wherein the second overhead stream is enriched withacetaldehyde with respect to the at least a portion of the firstoverhead stream; and (d) extracting the second overhead stream withwater to obtain an aqueous acetaldehyde stream comprising acetaldehydeand a raffinate comprising methyl iodide.
 16. The process of claim 15,further comprising condensing and biphasically separating the firstoverhead stream to form a light liquid phase and a heavy liquid phase,wherein the at least a portion of the first overhead stream distilled instep (c) comprises the light liquid phase.
 17. The process of claim 15,further comprising condensing and biphasically separating the firstoverhead stream to form a light liquid phase and a heavy liquid phase,wherein the at least a portion of the first overhead stream distilled instep (c) comprises the heavy liquid phase.
 18. The process of claim 15,wherein the acetic acid product comprises less than 250 wppm propionicacid.
 19. A process for producing an acetic acid product, comprising thesteps of: continuously reacting methanol with carbon monoxide in thepresence of a rhodium catalyst, an iodide salt, from 0.1 to 3.5 wt. %water, and methyl iodide to form a reaction medium, wherein the reactionis carried out at a temperature from 100 to 300° C. and a hydrogenpartial pressure from 0.3 to 2 atm, wherein the reaction mediumcomprises from 100 to 3000 wppm rhodium, separating the acetic acidproduct from the reaction medium; determining the butyl acetateconcentration in the acetic acid product; adjusting at least one of thetemperature, the hydrogen partial pressure and the rhodium catalystconcentration when the butyl acetate concentration in the acetic acidproduct is greater than 10 wppm; and removing acetaldehyde from a streamderived from the reaction medium to maintain an acetaldehydeconcentration of less than 1500 wppm in the reaction medium.